Stresses to plants may be caused by both biotic and abiotic agents. For example, biotic causes of stress include infection with a pathogen, insect feeding, parasitism by another plant such as mistletoe, and grazing by ruminant animals. Abiotic stresses include, for example, excessive or insufficient available water, temperature extremes, synthetic chemicals such as herbicides, and excessive wind. Yet plants survive and often flourish, even under unfavorable conditions, using a variety of internal and external mechanisms for avoiding or tolerating stress. Plants' physiological responses to stress reflect changes in gene expression.
Insufficient water for growth and development of crop plants is a major obstacle to consistent or increased food production worldwide. Population growth, climate change, irrigation-induced soil salinity, and loss of productive agricultural land to development are among the factors contributing to a need for crop plants which can tolerate drought. Drought stress often results in reduced yield. In maize, this yield loss results in large part from plant failure to set and fill seed in the apical portion of the ear, a phenomenon known as tip kernel abortion.
Low temperatures can also reduce crop production. A sudden frost in spring or fall may cause premature tissue death.
Physiologically, the effects of drought and low-temperature stress may be similar, as both result in cellular dehydration. For example, ice formation in the intercellular spaces draws water across the plasma membrane, creating a water deficit within the cell. Thus, improvement of a plant's drought tolerance may improve its cold tolerance as well.
CBF genes (for C-repeat/DRE binding factor) encode proteins which may interact with a specific cis-acting element of certain plant promoters. (U.S. Pat. Nos. 5,296,462 and 5,356,816; Yamaguchi-Shinozaki, et al., The Plant Cell 6:251-264 (1994); Baker, S. S., et al., Plant Mol. Biol. 24:701-713 (1994); Jiang, C., et al., Plant Mol. Biol. 30:679-684 (1996)) The cis-acting element is known as the C-repeat/DRE and typically comprises a 5-base-pair core sequence, CCGAC, present in one or more copies.
CBF proteins comprise a CBF-specific domain and an AP2 domain and have been identified in various species, including Arabidopsis (Stockinger et al., Proc. Natl. Acad. Sci. 94:1035-1040, 1997; Liu et al., Plant Cell 10:1391-1406, 1998); Brassica napus, Lycopersicon esculentum, Secale cereale, and Triticum aestivum (Jaglo et al., Plant Phys. 127:910-917, 2001) and Brassica juncea, Brassica oleracea, Brassica rapa, Raphanus sativus, Glycine max, and Zea mays (U.S. Pat. No. 6,417,428).
Overexpression of CBF in plants has been shown to improve tolerance to drought, cold, and/or salt stress (Jaglo-Ottosen et al., Science 280:104-106, 1998; Kasuga et al., Nature Biotechnology 17:287-291, 1999; Hsieh et al., Plant Phys. 129:1086-1094, 2002; Hsieh et al., Plant Phys. 130:618-626, 2002; Dubouzet et al., Plant J. 33:751-763, 2003). While CBF transcription factors may be useful in transgenic approaches to regulate plant response to stress, constitutive expression of CBF results in negative pleiotropic effects. Controlled expression of CBF in selected tissues and/or under stress conditions is of interest.